libvirt: start lxc from block device

Currently, lxc containers can be started only from an image.
Adding support to start lxc containers from block device volumes.

Separating the lxc disk handling code from _create_domain to _lxc_disk_handler
that will use block_device_mapping and will handle the pre and post lxc start
actions on the disk, both for images and volumes.

Implements blueprint libvirt-start-lxc-from-block-devices
Change-Id: Ia980f65384af2818f46d2e91df46f131b2ce3a77
Merge "libvirt: start lxc from block device"